A keyless entry system capable of locking and unlocking an automobile without insertion of the key into the keyhole is put to practical use. Japanese Patent Unexamined Publication No. H10-231651 discloses an energy-saving example of a keyless entry system having an answer-back function in which a portable device (so-called a remote controller) can confirm if the automobile has been locked or not.
It is desirable to reduce power consumption in receiving electromagnetic waves, also on the side of a lock mechanism (a device incorporated in an automobile, for example) of a keyless entry system. Especially, a keyless entry system in an automobile and the like is powered by a battery in many cases. Also from the viewpoint of efficiently utilizing the limited power source, reduction in power consumption is required. Japanese Patent Unexamined Publication No. 10-231651 discloses a method of turning off the system unless it is necessary, to reduce power consumption. However, if reception of electromagnetic waves is stopped on the side of the lock mechanism, users cannot lock or unlock the mechanism although they want to. This poses a problem of affecting convenience of the users.